Mention with reason the colour changes observed when:

(i) silver chloride is exposed to sunlight.

(ii) copper powder is strongly heated in the presence of oxygen.

(iii) a piece of zinc is dropped in copper sulphate solution.

Chemical Reactions and Equations

Types of Chemical Reactions

Official Answer

The observations, reasons, and chemical equations for the given reactions are:

(i) Silver chloride is exposed to sunlight:

Observation: The white silver chloride turns grey.
Reason: It undergoes photochemical decomposition in the presence of sunlight to form grey silver metal and chlorine gas.
Chemical Equation:

$2AgCl(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Cl_2(g)$

(ii) Copper powder is strongly heated in the presence of oxygen:

Observation: The reddish-brown copper powder turns black.
Reason: Copper reacts with oxygen in the air upon heating to form black copper(II) oxide via an oxidation reaction.
Chemical Equation:

$2Cu(s) + O_2(g) \xrightarrow{\text{Heat}} 2CuO(s)$

(iii) A piece of zinc is dropped in copper sulphate solution:

Observation: The blue colour of the copper sulphate solution fades and becomes colourless, and a reddish-brown deposit of copper is formed on the zinc piece.
Reason: Zinc is more reactive than copper, so it displaces copper from copper sulphate solution to form colourless zinc sulphate solution (displacement reaction).
Chemical Equation:

$Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$

photochemical decompositiongrey silveroxidationblack copper oxidedisplacementcolourless zinc sulphate

Marking Scheme

11.0 mark for part (i): 0.5 marks for correct observation (white to grey) and 0.5 marks for correct reason/equation.
21.0 mark for part (ii): 0.5 marks for correct observation (reddish-brown to black) and 0.5 marks for correct reason/equation.
31.0 mark for part (iii): 0.5 marks for correct observation (blue to colourless/reddish-brown deposit) and 0.5 marks for correct reason/equation.

Hint

Recall the effects of sunlight on silver halides, the oxidation of metals upon heating, and the reactivity series of metals.

Quick Oral Answer

When silver chloride is exposed to sunlight, it decomposes into grey silver metal and chlorine gas. When copper is heated, it oxidizes to black copper(II) oxide. When zinc is added to blue copper sulphate, it displaces copper to form colourless zinc sulphate.

Analysis & Explanation

This question tests the understanding of three fundamental types of chemical reactions:

Photochemical Decomposition: Silver chloride ( $AgCl$ ) is highly sensitive to light. Sunlight provides the activation energy to break the ionic bonds in $AgCl$ , yielding metallic silver ( $Ag$ ) which appears grey, and chlorine gas ( $Cl_2$ ).
Oxidation Reaction: Heating copper in air causes it to combine with atmospheric oxygen, forming copper(II) oxide ( $CuO$ ). The addition of oxygen to a substance is defined as oxidation, and the resulting oxide is black.
Displacement Reaction: According to the reactivity series, zinc ( $Zn$ ) is more reactive than copper ( $Cu$ ). When zinc is added to blue copper sulphate ( $CuSO_4$ ) solution, it displaces copper ions, forming colourless zinc sulphate ( $ZnSO_4$ ) and precipitating metallic copper as a reddish-brown deposit.

Common Mistakes

1Forgetting to write the balanced chemical equations with correct state symbols.
2Confusing the colour of copper oxide as brown instead of black.
3Stating that copper sulphate turns green instead of colourless when zinc is added (iron turns it green, but zinc turns it colourless).

Interesting Facts

The photochemical decomposition of silver halides was the fundamental chemical principle behind black-and-white photography.

Black copper(II) oxide can be reduced back to reddish-brown copper metal by passing hydrogen gas over the heated oxide.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Chemical Reactions and Equations" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Types of Chemical Reactions" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.

Mention with reason the colour changes observed when: (i) silver chloride is exposed to sunlight. (ii) copper powder is strongly heated in the presence of oxygen. (iii) a piece of zinc is dropped in copper sulphate solution.